WASHINGTON - Congress is getting an earful from angry airline customers who are furious with carriers that cancel flights at the last minute because they don't have enough passengers aboard to turn a profit.

The airlines customarily deny that they cancel flights for economic reasons and, instead, often blame cancellations on bad weather, mechanical failures or time-maxed flight crews - all assertions that are nearly impossible for passengers to challenge.

Passenger rights advocates are asking lawmakers to step in and clamp down on airlines.

Rep. Dan Lipinski, D-Ill., is among lawmakers who suspect financial motives lead the airlines to cancel half-full flights.

"It seems flights that are canceled for `mechanical reasons' are the ones that are least full," Lipinski said in an interview.

Sen. John Thune, R-S.D., is pushing legislation that would require airlines to publish the percentage of flights that they cancel and flag chronically canceled flights.

Sen. Olympia Snowe, R-Maine, said she is hearing complaints about airlines "summarily canceling flights without any explanation."

"This can ruin the plans of consumers who have spent a lot of money," Snowe says. "The airlines have a responsibility to make sure they are responsive and accountable to the consumer."

Congress is looking broadly at the issue of airline accountability as lawmakers prepare to reauthorize the Federal Aviation Act, which expires in September.

David Castelveter, a spokesman for the Air Transport Association, the trade group for major U.S. carriers, bristles at the suggestion that airlines dump passengers if a particular flight doesn't make economic sense.

"An airline does not cancel a flight because an airline isn't going to make money on a flight," Castelveter insists. But he acknowledges that carriers "do cancel flights because they need an airplane on another route."

He says carriers "swap out planes if one is booked and another is part empty."

"If you take away the ability for a carrier to decide how it's going to use its airplanes, that is a recipe for disaster," Castelveter says.

Castelveter says when airlines cancel a flight because they need the plane elsewhere, airlines refund the tickets or make alternative arrangements - an assertion disputed by consumer groups.

Advocates for jilted passengers say it's tough to quantify how many passengers get dumped for economic reasons because the airlines are the primary keepers of those statistics.

Bill Mosley, a spokesman for the Department of Transportation, says any airline that made a practice of canceling flights for economic reasons could be in violation of the clause in Federal Aviation Administration law that prohibits "unfair" or "deceptive" practices.

"We'd have to investigate," Mosley says, adding that the department doesn't pursue these "economic cancellations" unless the agency were to find "a pattern of violations by a carrier."

There aren't enough complaints to warrant more attention, he said. The department reported only 28 complaints regarding economic cancellations in 2005, 17 complaints in 2006 and eight this year.

Paul Hudson, executive director of the Aviation Consumer Action Project, an advocacy group for air travelers and a member of the FAA rule-making advisory committee, says carriers won't admit they cancel flights for economic reasons because that would amount to a breach of contract since airline "contracts of carriage" usually say they will use their best efforts to get passengers to their destinations.